First Results from Hollow-Channel Plasma Wakefield Acceleration Experiments with Positron Beams at FACET*

We report on the first results from a hollow-channel plasma wakefield acceleration experiment using positron beams at FACET. A meter-scale plasma channel is created by field ionizing lithium vapor using an intense laser pulse that has a transverse J8 Bessel profile. The plasma channel is roughly 600 $\mu$m in diameter and has unionized vapor at its center. A 20.35 GeV positron beam with spot size of roughly 50 $\mu$m was sent through the channel. We observed the transverse beam profile while varying the position of the beam relative to the channel. Our measurements clearly indicate that a plasma channel was formed. We characterize the strength of the wake and discuss plans for subsequent experiments.